_Helium._ A gaseous element. Symbol: He. _Henry._ The unit of inductance. The inductance in a circuit in which the induced electromotive force is one volt when the inducing current varies at the rate of one ampere per second. _Herb._ A plant whose stem is not woody or persistent. _Hermaphrodite._ An organism possessing male and female characteristics or organs. _Herpetology._ The study of reptiles. _Hexapoda._ The order of insects. _Hilum._ The point where the seed is attached to the seed-vessel. _Hinterland._ The district behind a coast. _Histology._ The study of organic tissues, usually with the aid of a microscope. _History._ The study of the past. It deals with human institutions rather than with the animal, vegetable, or mineral kingdom. History is concerned first of all with concrete data, specific cases, and then with explaining them in the light of general laws. There have been some ingenious, but not altogether successful, attempts to make history into a science. _Holmium._ A metallic element. Symbol: Ho. _Homo-._ In compounds: same, similar. _Homology._ That relation between parts which results from their development from corresponding embryonic parts, either in different animals or in the same individual. _Hormone._ A stimulating substance carried from one organ to another. _Horse-power._ The unit of power in the English system. It is equal to 550 foot pounds per second. _Humidity._ Moisture. _Hurricane._ A wind of stormy violence. _Hybrid._ The offspring of two plants or animals of different sorts. _Hydrate._ A compound containing combined water. _Hydraulics._ The science dealing with liquids in motion. _Hydrocarbon._ A compound of carbon and hydrogen. _Hydrogen._ A gaseous element. Symbol: H. _Hydrolysis._ The decomposition of water in such a reaction as includes the formation of new compounds with the hydrogen and the oxygen of the water. _Hydrometer._ An instrument used to determine the specific gravity of a liquid. _Hydrostatics._ The science which deals with the pressure of liquids. _Hydrous._ Containing water, whether in combination or in a mixture. _Hydrozoan._ A member of a class (Hydrozoa) of coelenterate animals, chiefly marine, including jellyfish. _Hygiene._ The science of sanitation, preventive medicine. In the United States, textbooks of hygiene are chiefly concerned with proving that alcohol and tobacco are injurious. Here is an example of “science” created by legislative fiat. _Hygrometry._ The measuring of atmospheric moisture. _Hypothesis._ A supposition used as a basis for reasoning, a provisional explanation. If it appears to be unconfirmed by the facts, it is rejected. Otherwise it becomes a theory, then a law. _Igneous._ Formed by great heat, as rocks. _Indehiscent._ Not bursting open at maturity. _Indium._ A metallic element. Symbol: In.